1. Field of the Invention
The present invention relates to a radar system for detecting a target with radio waves.
2. Description of the Related Art
FM-CW (frequency-modulated continuous-wave) radar systems have been developed as in-vehicle radar systems, which are mounted on mobile bodies such as vehicles, for detecting, for example, other vehicles, human beings, and obstructions.
An FM-CW radar system transmits a frequency-modulated continuous-wave signal as a transmission signal, mixes a reception signal (transmission signal reflected from a target) and the transmission signal to generate a beat signal, and detects the relative position and the relative speed of the target based on the frequency of the beat signal.
Circuits for generating the transmission signal include a voltage controlled oscillator (hereinafter referred to as a VCO) in which an oscillation frequency varies in accordance with a control voltage. Since the relationship between the control voltage and the oscillation frequency in the VCO is known, it is possible to perform the desired frequency modulation by controlling the control voltage applied to the VCO.
However, the characteristic of the oscillation frequency with respect to the control voltage applied to the VCO is not necessarily the same for every VCO. Also, the characteristic varies with temperature.
In order to solve the above problems, prior art reference (1), Japanese Unexamined Patent Application Publication No.7198833, discloses a radar system that performs nonlinear correction of a VCO by pre-storing data relating to the control voltage applied to the VCO in a memory and reading the stored data, and prior art reference (2), Japanese Unexamined Patent Application Publication No.10-197625, discloses a radar system that stores in a memory data relating to a correction voltage used for correcting the control voltage applied to a VCO in accordance with the temperature and corrects the control voltage in accordance with the temperature of the VCO.
However, in the radar system disclosed in prior art reference (1), a large memory is required for storing the corrected control voltage data. In the radar system disclosed in prior art reference (2), since the temperature data is also stored, a much larger memory is required. Thus, the circuit size disadvantageously increases, resulting in very high cost for these known radar systems.
In order to overcome the problems described above, preferred embodiments of the present invention provide a compact radar system having reduced memory space at a much lower cost.
In addition, preferred embodiments of the present invention provide a method of adjusting characteristics of a radar system in which the linearity in the rising phase (period) and the falling phase (period) of a transmission frequency varying with time can be corrected for the entire radar system without being affected by variations in individual components.
According to a preferred embodiment of the present invention, a radar system includes a transmitting/receiving unit for repeatedly transmitting a triangular transmission signal including an up-modulation interval when a frequency increases gradually and a down-modulation interval when a frequency decreases gradually, and for receiving a reception signal including a reflected signal from a target. The transmitting/receiving unit has a voltage controlled oscillator for determining a transmission frequency, and a voltage signal for frequency modulation being supplied to the oscillator. The radar system also includes a frequency analysis unit for calculating data concerning the frequency spectrum of a beat signal that has a frequency that is substantially equal to the difference between the frequency of the transmission signal and that of the reception signal, a data storing unit for storing time-varying characteristics of input values to a D/A converter that monotonically change an oscillation frequency of the voltage controlled oscillator with time, in the form of data concerning an expression (formula) representing the time-varying characteristics, and a data processing unit for determining the input values to the D/A converter with reference to the data. The radar system detects at least one of the relative distance to the target and the relative speed thereof based on the beat signal during the up-modulation interval and the beat signal during the down-modulation interval.
Such a structure reduces the amount of data representing the time-varying characteristics of the input values to the D/A converter, thereby realizing a compact radar system at a lower cost.
The expression may be a polynomial expression and the data concerning the expression may be a coefficient of each term of the polynomial expression. For example, use of a cubic polynomial expression (a cubic equation) can increase the accuracy at which the oscillation frequency of the voltage controlled oscillator monotonically varies with time.
The data processing unit preferably performs time integration of differential equations given by the first-order differentiation to the nth-order differentiation, where n represents the order of the polynomial expression, to determine the time-varying input values to the D/A converter. In this case, the data can be processed using only addition and subtraction, and without using multiplication or division. Accordingly, simple logic circuits, CPLDs, FPGAs and so on can be used instead of high-speed microcomputers or DSPs, thereby reducing the cost.
It is preferable that the expression be provided at various temperatures and the data processing unit refer to an expression corresponding to the detected temperature to determine the time-varying input values to the D/A converter. Thus, the characteristics become steady across a wider temperature range.
It is also preferable that the expression be provided at several discrete temperatures and the data processing unit determine data corresponding to the detected temperature by an interpolation or extrapolation method. In such a case, higher-accuracy correction with respect to temperature can be achieved with a reduced amount of data concerning the expression at various temperatures.
A radar system preferably includes a counter for monotonically counting a clock signal having a predetermined frequency incrementally during the up-modulation interval and decrementally during the down-modulation interval. The data processing unit preferably determines the input values to the D/A converter in accordance with the count value of the counter. Such a structure allows for determination of the time-varying input values to the D/A converter during the up modulation interval and also allows for the down-modulation interval to be determined by using a single expression, thereby reducing the circuit size.
In another preferred embodiment of the present invention, a method of adjusting characteristics of the radar system includes the steps of measuring the frequency of the transmission signal with, for example, a spectrum analyzer while the input values to the D/A converter are being varied to determine the relationship between the input values to the D/A converter and the frequency of the transmission signal, determining the data concerning the expression representing the time-varying characteristics of the input values to the D/A converter from the relationship, and storing the data in the data storing unit.
In this method, the characteristics can be properly adjusted for every radar system by determining time-varying characteristics of the input values to the D/A converter that monotonically change the oscillation frequency of the voltage controlled oscillator with time.
The above and other elements, features, characteristics and advantages of the present invention will become clear from the following description of preferred embodiments taken in conjunction with the accompanying drawings.